It is well known in the art to utilize an electron gun within a traveling-wave tube (TWT) or other charged particle device such as a linear accelerator, a free electron laser, a switch tube or a crossed-field tube. A TWT, in particular, is a broad-band, microwave tube which depends for its characteristics upon interaction between the electric field of a wave propagated along a wave guide and a beam of electrons traveling with the wave. In this tube, the electrons in the beam travel with velocities slightly greater than that of the wave, and, on the average, are slowed down by the field of the wave. Thus, the loss in kinetic energy of the electrons appears as an increased energy conveyed by the field to the wave. The TWT therefore, may be used as an amplifier or as an oscillator.
The electron gun which forms the heart of the TWT is typically formed with a cathode and anode between which are disposed grids. An electron gun showing such an arrangement may be found in prior U.S. Pat. No. 3,558,967, issued Jan. 26, 1971, by George V. Miram. The Miram patent utilizes a control grid and a shadow grid having the same pattern for the purpose of selectively blocking electron flow from the cathode to the control grid thereby preventing excessive heating of the control grid by electron bombardment. The shadow grid placed adjacent to the cathode causes distortion of the electric fields. This creates electron trajectories in the beam of electrons flowing from the cathode toward the anode to cross over one another and diverge from the desired laminar flow. Such crossing trajectories create serious heating problems when the stray electrons strike parts of the microwave tube structure downstream from the electron gun. The Miram reference overcomes this defocusing problem by either imbedding the shadow grid within the cathode or recessing the shadow grid in a recessed pattern within the surface of the cathode.
When the shadow grid is imbedded within the cathode, the result is a serious shortening of the cathode life due to the poisoning of the cathode by the contacting grid or due to grid emission resulting from migration of the emissive material onto the grid. The second Miram solution is to recess the grid in a noncontact manner within square cornered grooves in the surface of the cathode. In either solution that the Miram reference teaches, the spacings are impractically small. These small spacings provide less than optimum electron optics. Furthermore, the Miram reference teaches the need for relieving the surface of the cathode to form dimples between the recessed shadow screen. These dimples, or secondary concaved surfaces, are intended to form tiny beamlets which are ultimately focused into a single unitary linear beam after passage through the shadow and control grids.
One disadvantage of forming dimples, or secondary concaved emitter surfaces, within the concaved surface of the cathode is the added fabrication steps required. Further, each dimple must be symmetrical about its center. Thus, the pattern of the shadow grid and accompanying control grid or grids is needlessly complicated in order to match the symmetry of the dimpled pattern. This requires tighter grid tolerances and creates alignment problems. Finally, the pattern of grooves on the cathode surface is unnecessarily complex and difficult to manufacture.
After the suggested use of an imbedded shadow grid, Miram taught the use of a spherically-concaved and dimpled cathode surface, together with a pair of axially-spaced, spherically-concaved, focus-and-control grids in his coinvention, U.S. Pat. No. 3,983,446, which issued Sept. 28, 1976. Other U.S. patents which show grooved control grids may be found in U.S. Pat. No. 3,500,107 which issued Mar. 10, 1970, by J. E. Beggs and U.S. Pat. No. 2,977,496 which issued Mar. 28, 1961 by H. D. Doolittle. These patents show a grooved, spherical, cylindrical or flat-surfaced cathode, respectively. Except for the flat-surfaced cathode shown in the Beggs patent, the curved cathode surfaces are each shown with secondary curved surfaces that are difficult to machine or otherwise fabricate.
A copending patent application, Ser. No. 362,790, filed Mar. 28, 1982, by Richard B. True, entitled Improved Dual-Mode Electron Gun, assigned to the same assignee as the present invention, shows the use of a smooth, concaved cathode in a dual-mode electron gun. However, this reference used a shadow grid with two distinct patterns of conductive elements and a varying potential to accomplish its dual-mode function. It does not teach an improved cathode and shadow grid configuration.